1. Field of the Invention
The present invention relates to a radiation field characteristic measuring apparatus, and more particularly, to an apparatus for measuring radiation field characteristics of a radio communication device and the like, with respect to the total solid angle thereof.
2. Description of the Related Art
Recently, radio pagers, cordless telephones, and other portable radio communication devices have been quickly changed into miniature high-function versions. The communication devices of this type are often provided with built-in antennas. In these devices, the characteristics of the built-in antennas are greatly influenced by changes of external environments, such as the conditions of electronic parts. Thus, it is very difficult to develop the built-in antennas independently. The radiation efficiency of the built-in antennas of the communication devices may be considerably lowered due to loss caused by the surrounding electronic parts or users' bodies. In developing the built-in antennas, therefore, they must be designed so that the lowering of their radiation efficiency is minimized Although the input impedance of small antennas can be adjusted relatively easily, it is often difficult for them to realize desired radiation pattern and radiation efficiency. Thus, the built-in antennas must be studied and developed in consideration of the influence of the surrounding electronic parts and the like. There may be two approaches, theoretical and experimental, to the design of the built-in antennas covering these circumstances. In actual product development, however, it is extremely hard to theoretically analyze the radiation characteristics including the influence of the complicated electronic parts, such as printed boards. Naturally, therefore, the experimental approach is required for the analysis of these effects. In order to improve the antenna radiation efficiency by the experimental method, it is necessary to establish a method of antenna radiation efficiency measurement in which the radio communication devices with a built-in antenna can be tested in their actual working state, without the additional use of a measuring cable or the like. In consequence, there is a demand for the development of a measurement method which permits the measurement in the actual working state.
Theoretically, the radiation efficiency of an antenna can be obtained by integrating its radiation field with respect to its total solid angle. Practically, however, it has been regarded as difficult to measure radiation field characteristics of a tested object with respect to the total solid angle, in the VHF or UHF band within which most portable antennas are used. To the knowledge of the inventor hereof, no experimental results on such measurement have hitherto been reported. Thereupon, several other measurement methods for antenna radiation efficiency have been proposed. They include the Wheeler cap method and the Q-factor method, which are based on the measurement of the input impedance of the antenna, and the random field measurement method in which the tested object can be tested in its actual working state. Wheeler proposed the method (Wheeler cap method) in which a miniature antenna is covered by a method cap with a radius of about .lambda./6, and the radiation efficiency of the antenna is obtained by measuring the input impedance. Further, E.H. Newman et al. comparatively examined antenna efficiency measurement methods for a multiturn loop antenna based on the Wheeler cap method and the Q-factor method. Basically, these two methods are methods for obtaining the antenna radiation efficiency through the measurement of the antenna's input impedance, so that they can be easily applied to monopole antennas which can be set on a ground plane. In an actual radio communication device, however, the tested object must be fitted with some additional members, e.g., a coaxial cable for impedance measurement, a balun, etc. In measuring low-efficiency antennas with the radiation efficiency of -20 to -30 dB, such as those for radio pagers and the like, in particular, the balun and the measuring cable used are much larger in size than the built-in antenna. Accordingly, radiation from the cable and the balun greatly influences the measurement accuracy. Also, the efficiency measurement for the antennas of this type is influenced by the accuracy of an impedance measuring equipment.
J.B. Andersen proposed the method (random field measurement method or RFM) in which the effective radiation efficiency of an antenna is obtained from the median of the values of power received by tested portable antennas used in an urban district. The RFM method is an easy method in which measurement can be made in consideration of the influence of users' bodies, without requiring any such large-scale equipment as an anechoic chamber. Requiring outdoor operations, however, this method is subject to some drawbacks; long measuring time, great transmission power, hard operation under bad weather, jamming by external electric waves, etc. In an indoor RFM using radio wave scatterers, as an improved version of the RFM, measurement can be made relatively easily in ordinary laboratories or plants, without attaching any measuring cable or the like to the radio communication device in an actual working state. According to this method, however, the DDD-value is not low in every where, therefore the measuring point must be changed for equalization. Thus, there had not yet been developed measuring apparatuses which can measure radiation characteristics with respect to the total solid angle. In order to seek for the radiation efficiency of the tested object, therefore, a lot of studies have been made.
Meanwhile, new technology, such as the diversity system, will be introduced into future shifting communication, so that it is essential to grasp the radiation directivity of the tested object. Since the radiation directivity of a built-in antenna is different from that of an independent antenna, moreover, the radiation characteristics of the antenna cannot be accurately evaluated by only measuring its specific cut pattern and polarized waves. Thus, in studying built-in antennas, it is important to detect the radiation efficiency and directivity of the object quickly and accurately. Nevertheless, no such measuring apparatuses have yet been reported. Accordingly, it has been impossible to obtain correlation factors for the effective gain and diversity, based on the distribution of the arrival angle of electromagnetic waves, as an essential evaluation index for antennas for moving bodies.
The radiation field characteristics of a tested device with respect to the total solid angle thereof are the characteristics of electromagnetic fields at every point on a sphere or closed curved surface 11 with radius r around tested device 10, as shown in FIG. 1. These radiation characteristics are measured by using means which is equivalent to an arrangement of the tested device at each point on surface 11. More specifically, as shown in FIG. 2, device 10 is rotated around azimuth axis 13 and elevation axis 14, extending at right angles to each other, by means of rotating device 12, as shown in FIG. 2. On the other hand, receiving antenna 15 is situated at a predetermined distance from device 10, and electromagnetic waves received by antenna 15 are signal-processed by means of measurement control device 16. Thus, the radiation field characteristics with respect to the total solid angle are measured.
Conventionally, the rotating device for rotating the tested device is formed of metal. If metallic parts are likely to cause electric waves to scatter, thereby disturbing the measurement results, they are covered by means of a radio wave absorber to prevent the metal from influencing the measurement. If the frequency of the electromagnetic waves used is relatively high, e.g., within the microwave frequency band, the gain of the tested antenna is high, and the directivity is narrow or sharp. Therefore, a satisfactory measurement can be made by setting the metallic parts off the antenna beam. In the case of measuring the characteristics of electromagnetic waves with a relatively low frequency, e.g., within the VHF or UHF band, on the other hand, the following awkward situations will be caused. The rotating device includes a turntable for supporting the tested object and a supporting and rotating mechanism for supporting and rotating the turntable, which are formed of metal, as mentioned before. If the antenna size is fixed, moreover, the antenna gain for the VHF or UHF band is lower than that for the microwave frequency band. Thus, the directivity is wide enough to be applied to a microwave frequency band antenna. In consequence, it is difficult to prevent the antenna beam from being directed toward the metallic parts.
Accordingly, the inventor hereof proposed to improve the rotating device suited for microwaves into one suited for electromagnetic waves within the VHF or UHF band.
More specifically, in order to prevent the electromagnetic waves within the VHF or UHF band from being absorbed or scattered, the inventor hereof proposed to cover one lateral face of the turntable supporting and rotating mechanism by means of a radio wave absorber. In a region of the mechanism in one direction thereof, however, electromagnetic waves emitted from the tested device are absorbed by the radio wave absorber, so that the radiation field characteristics cannot be measured. Since the wavelength of the electromagnetic waves within the VHF or UHF band is relatively long, furthermore, the radio wave absorber must be as tall as about 1 m. Accordingly, there will be a very wide region in which the electromagnetic waves emitted from the tested device are absorbed by the radio wave absorber, and the field characteristics cannot therefore be measured. Thus, the conventional rotating device for microwaves cannot be improved into the one suited for the electromagnetic waves within the VHF or UHF band. This will be described more specifically with reference to a prior art arrangement.
An example of a prior art radiation field characteristic measuring apparatus is mentioned in "Toyo Technica Electronic Measuring Instrument Catalog" (1985), on page 708. In this conventional apparatus (Scientific-Atlanta, Inc. USA Model Tower 58710A series), a turntable for supporting a tested object (e.g., a scale model of an air plane furnished with an antenna) is mounted on the distal end of an azimuth shaft, whose proximal end is rotatably supported on an azimuth post by means of a bearing. The proximal end of the azimuth post is supported in a position eccentric to the center of rotation of an azimuth pedestal, which is supported on an elevation shaft. With this arrangement, the radiation field characteristics can be measured while rotating the tested object on the turntable around azimuth and elevation axes which extend at right angles to each other.
This prior art apparatus is used with a microwave frequency band, and the turntable, bearing, and pedestal are made of metal. Power is transmitted by means of a metallic propeller shaft. Thus, if this apparatus is used with a relatively low frequency band, such as the VHF or UHF band, the following problems arise. In the VHF or UHF band, the radiation characteristics of the tested object are not so sharp as in the microwave frequency band, so that the electromagnetic waves are reflected or scattered by the metallic parts. Thus, a high-frequency current flows through the turntable and the elongated propeller shaft. It is necessary, therefore, to use a radio wave absorber to cover the front portion of the metallic turntable which supports the tested object. In the region behind the turntable with respect to the tested object, therefore, the electromagnetic waves from the object are absorbed, so that the radiation field characteristics cannot be measured at all. With the VHF or UHF band, in particular, the thickness of the wave absorber must be as great as 1 m in consideration of the wavelength, so that unmeasurable regions are too wide to be practical.
Accordingly, there is a demand for the development of an apparatus which can highly accurately measure the radiation field characteristics of a tested device with respect to the total solid angle thereof. Such an apparatus is required particularly for a VUHF-band antenna with a wide beam angle.
In a conventional antenna whose wave arrival probability is not constant with respect to the total solid angle thereof, moreover, its evaluation index is obtained without considering distribution of the arrival angle of radio waves. Thus, the evaluation index of the antenna obtained is not suited for the actual radio conditions. This is because the radiation characteristics of the tested object cannot be measured with respect to the total solid angle.